Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82072, USA.
Department of Geography, University of Oregon, Eugene, Oregon 97403, USA.
Nature. 2018 Jan 31;554(7690):92-96. doi: 10.1038/nature25464.
Cooling during most of the past two millennia has been widely recognized and has been inferred to be the dominant global temperature trend of the past 11,700 years (the Holocene epoch). However, long-term cooling has been difficult to reconcile with global forcing, and climate models consistently simulate long-term warming. The divergence between simulations and reconstructions emerges primarily for northern mid-latitudes, for which pronounced cooling has been inferred from marine and coastal records using multiple approaches. Here we show that temperatures reconstructed from sub-fossil pollen from 642 sites across North America and Europe closely match simulations, and that long-term warming, not cooling, defined the Holocene until around 2,000 years ago. The reconstructions indicate that evidence of long-term cooling was limited to North Atlantic records. Early Holocene temperatures on the continents were more than two degrees Celsius below those of the past two millennia, consistent with the simulated effects of remnant ice sheets in the climate model Community Climate System Model 3 (CCSM3). CCSM3 simulates increases in 'growing degree days'-a measure of the accumulated warmth above five degrees Celsius per year-of more than 300 kelvin days over the Holocene, consistent with inferences from the pollen data. It also simulates a decrease in mean summer temperatures of more than two degrees Celsius, which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records. Despite the differing trends, pollen- and marine-based reconstructions are correlated at millennial-to-centennial scales, probably in response to ice-sheet and meltwater dynamics, and to stochastic dynamics similar to the temperature variations produced by CCSM3. Although our results depend on a single source of palaeoclimatic data (pollen) and a single climate-model simulation, they reinforce the notion that climate models can adequately simulate climates for periods other than the present-day. They also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11,000 years.
在过去的两千年中，冷却现象得到了广泛的认可，并被推断为过去 11700 年（全新世）全球温度趋势的主导因素。然而，长期冷却现象与全球驱动力一直难以协调，气候模型始终模拟出长期变暖的情况。模拟结果与重建结果之间的差异主要出现在北半球中纬度地区，这些地区的海洋和沿海记录使用多种方法推断出明显的冷却现象。在这里，我们表明，从北美和欧洲的 642 个地点的亚化石花粉重建的温度与模拟结果非常吻合，并且在大约 2000 年前，长期变暖而非冷却定义了全新世。重建结果表明，长期冷却的证据仅限于北大西洋记录。大陆上的全新世早期温度比过去两千年低了 2 摄氏度以上，这与气候模型共同体气候系统模型 3（CCSM3）中模拟的残留冰盖的气候效应一致。CCSM3 模拟出整个全新世时期“生长度日”（每年积累的 5 摄氏度以上的热量）增加了 300 多开尔文日，这与花粉数据的推断一致。它还模拟出夏季平均气温下降了 2 摄氏度以上，这与重建的海洋趋势相关联，突出了记录中不同亚季节敏感性的潜在重要性。尽管存在不同的趋势，但花粉和海洋重建在千年至百年的时间尺度上是相关的，这可能是对冰盖和融水动态以及类似于 CCSM3 产生的温度变化的随机动态的响应。尽管我们的结果取决于单一的古气候数据来源（花粉）和单一的气候模型模拟，但它们增强了气候模型可以充分模拟除当前时期以外的其他时期的气候的观念。它们还表明，近几十年来的放大变暖使温度升高到过去 11700 年中任何一个世纪的平均值以上。
